Why Seed oils are unhealthy

If there is one dietary change that has the most impact, and which most people aren’t aware of, it’s to remove seed oils from the diet. Everyone knows sugar and processed junk “foods” are bad, but seed oils are ubiquitous and I see them everywhere, including in so called “healthy” packaged foods and restaurants. The evidence is pretty solid, so I don’t know why it hasn’t been communicated in mainstream news/health advice like the harms of sugar and trans fat have, but I really hope it does one day, so restaurants and food manufacturers can provide healthier food. Until then, it is up to the individual to make their own food and read labels diligently. Here, I’ll break down what seed oils are, why they’re so bad, and what to replace them with.

What are seed oils?

Seed oils are generally oils produced from seeds/nuts, such as sunflower oil, rapeseed/canola oil, peanut oil, corn oil, soybean oil etc. They are often labelled “vegetable oil” on labels to make them sound healthier. They were popularized during the time of the (completely unfair) demonization of animal fats and marketed as a “heart healthy alternative”, which has resulted in a lot of damage to public health. The tide is slowly turning, however.

Why are seed oils so bad?

They generally contain large proportions of polyunsaturated fats which are the least stable and most prone to oxidation. The main types of polyunsaturated fats are omega-3s and omega-6s. Most people know how important omega 3s are for general health, but both types are considered ‘essential’ as you can only get them from diet. The problem is that we evolved eating omega 3:6 in a ratio of about 1:1 to 1:4 (which appears to be optimal), but because of the massive increase in seed oil consumption in modern society, most people’s ratios are about 1:15/16. Excessive omega 6’s (especially linoleic acid which is ubiquitous in seed oils) cause all sorts of health problems, detailed below. These fats should generally never be heated, but even before they make it to kitchens, many of these oils are heated/refined/deodorized or processed in some other ways, so they are already oxidised and highly damaged before they get to a kitchen. These fats get incorporated into your cell membranes, and can wreak havoc in the body. Even if you are eating cold pressed nut oils, you have to be careful with how much polyunsaturated fats you are eating (whether omega 3 OR omega 6). A little is essential, too much is harmful, and most people would do well to increase their omega 3 intake (as I’ve discussed before), and reduce omega 6 intake.

Here are just a few studies that indicate just how problematic overconsumption of seed oils can be..

  • Mothers with higher levels of linoleic acid (LA) levels in breastmilk had children who scored lower on motor and cognitive tests at age 2-3 years old. Breastfeeding is generally known to improve cognition compared to formula feeding, but in this study those with the highest levels of LA scored closer to children who had never been breastfed.
  • Overconsumption of omega 6 fats can contribute to chronic pain and peripheral nerve damage. This study was done in mice, but it found that accumulation of omega 6 fats in cells can enhance pain sensitivity and cause histologic markers of periphal nerve damage. These changes were reversed by switching to omega 3s. The study also looked at humans, and found that type 2 diabetics with higher levels of omega 6 levels in the skin had worse reported pain scores and took more pain relieving medication
  • High omega 6 intake leads to low grade chronic inflammation which can contribute to wide ranging effects in the body including cardiovascular disease
  • Higher serum polyunsaturated fats are linked to more inflammation and symptom severity in Inflammatory Bowel Disease in humans and also increases IBD associated colon cancer in mice
  • Excessive linoleic acid intake has been implicated in obesity
  • A recent study in male mice showed that soybean oil disrupted gene expression in the hypothalamus (a brain region that regulates metabolism, temperature, reproduction, stress and other vital processes) resulting in decreased oxytocin (the “bonding hormone”) and caused glucose intolerance. Coconut oil did NOT have these effects.
  • A study in the US found that soybean and canola oils contained between 0.56% to 4.2% trans fats which are normally formed in partially hydrogenated fats and are well known to be some of the most harmful fats
  • Oxidised fats (which occur mainly in seed oils) have been shown to promote tumour formation and artherosclerosis in animals, as well as embryo malformations when fed to pregnant rats.

There are many, many more studies showing adverse effects in both mice and humans. The other reason to avoid seed oils is that they are an extremely recent introduction to the human diet, and were not eaten by our hunter-gatherer ancestors (have you ever tried squeezing oil out of a soybean?). The Primal mantra is to first look at what humans have evolved with vs what is evolutionary new and then look at what scientific studies say.

So which oils should you cook with?

In general, the least refined and highest in monounsaturated and saturated fatty acids are the most stable and the healthiest. Studies have shown that seed oils are the highest in harmful oxidation products when heated. The graph below from a study done for a BBC show “Trust me, I’m a doctor”, is a good pictorial representation of which oils are safest when used for cooking (at the top) and which are the least safe.

The following is a list of the oils that I consume, because they are generally higher in saturated and monounsaturated fats, and also contain antioxidants that protect the oil from damage:

  • Coconut oil
  • Ghee
  • Beef tallow
  • Red palm oil
  • Sustainable palm oil
  • Extra virgin olive oil
  • Avocado oil

Once you have bought your healthy and ideally fresh oil, how do you take care of it at home before it enters your body? This a good guide on how to prevent oxidation in oils; the main things to do are to avoid high temperatures when cooking (and definitely avoid reusing oil), keep oils in the dark, and don’t leave lids open (which expose the oil to oxygen). Switching your cooking oils to healthier alternatives is one of the easiest and most effective strategies for your health but it can be difficult to avoid them when eating out; I’ve found that higher quality restaurants who make food from scratch are generally able to cook some menu items in olive oil or butter if requested. By asking food manufacturers/restaurants about the oils they use, questioning and sharing evidence, my hope is that more people will become knowledgeable about the dangers of these ultra processed “foodstuffs” and make the easy changes necessary for health!


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